They are one of the favorites for sure.
Lazio and Athletic Bilbao are two of the strongest teams in the league. Manchester United and Tottenham didn't perform like them. In the end it's still in group stage so don't come to conclusions easily. As long as they advance to quarter final there will be no problem.
Considering Lazio and Athletic Bilbao are in first and runner up positions in the qualifying table of this competition, it is natural that these two teams are favorites at least they are certain to advance to the next phase. the results they achieved in their six matches are quite good, both teams won five matches and drew one match. if we compare it with Manchester United of course the two teams above are much more consistent, as well as Spurs this team is also less favored when compared to Lazio and Athletic. the problem is this is just the early stages, which means that neither Lazio nor Bilbao can be sure to advance smoothly to the final round of this competition. after all there are still several teams ready to be their challengers, Lyon, Frankfrurt, Galatasaray or even other teams might block their progress in the knockout round later. we'll see after this qualifying phase is over, because the real competition will be more exciting than the qualifying round.